Microbeam analysis -- Analytical electron microscopy -- Method for the determination of energy resolution for electron energy loss spectrum analysis

This document specifies a determination procedure of energy resolution in the scanning transmission electron microscope or the transmission electron microscope equipped with the electron energy loss (EEL) spectrometer. This document is applicable to both in-column type EEL spectrometer and post-column type EEL spectrometer. These EEL signal detecting systems are applicable to a parallel detecting system and a serial detecting system.

Analyse par microfaisceaux -- Microscopie électronique analytique -- Méthode de détermination de la résolution énergétique pour l'analyse spectrale de la perte d'énergie des électrons

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Publication Date
22-Apr-2021
Current Stage
6060 - International Standard published
Start Date
27-Mar-2021
Completion Date
23-Apr-2021
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INTERNATIONAL ISO
STANDARD 23420
First edition
2021-04
Microbeam analysis — Analytical
electron microscopy — Method for the
determination of energy resolution
for electron energy loss spectrum
analysis
Analyse par microfaisceaux — Microscopie électronique analytique
— Méthode de détermination de la résolution énergétique pour
l'analyse spectrale de la perte d'énergie des électrons
Reference number
ISO 23420:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 23420:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 23420:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 3

5 Definition of the energy resolution ................................................................................................................................................... 5

6 Reference materials and energy determination ................................................................................................................. 5

6.1 General ........................................................................................................................................................................................................... 5

6.2 Materials selection for energy scale calibration ........................................................................................................ 5

6.3 Binding energy measurement of graphite in the XPS ............................................................................................ 6

7 Measurement procedure and energy resolution determination ....................................................................... 6

7.1 General ........................................................................................................................................................................................................... 6

7.2 Predetermination of binding energy .................................................................................................................................... 8

7.2.1 Obtain graphite and the other reference sample ................................................................................. 8

7.2.2 Measure C1s of graphite by using the XPS ................................................................................................. 8

7.3 Setup of the S/TEM and the EELS, and sample setting ........................................................................................ 8

7.4 First energy step, δE , calibration .......................................................................................................................................... 8

7.4.1 EELS acquisition set-up ............................................................................................................................................. 8

7.4.2 Determining the EELS first energy step, δE ............................................................................................

1 8

7.4.3 Acquisition of carbon K-edge EEL spectrum ........................................................................................... 9

7.4.4 Calculate calibrated energy step δE ............................................................................................................

1C 9
7.5 Measurement of peak close to the zero-loss peak, E , for the other reference
CZLP

sample using energy step δE ...................................................................................................................................................

1 12
7.5.1 EEL spectrum acquisition of the second reference sample using energy

step δE ...................................................................................................................................................................................

1 12

7.5.2 Obtain the value for CH between the zero-loss peak and the peak E ....................

2 CZLP 13

7.5.3 Calculate the peak E energy .................. .....................................................................................................13

CZLP

7.6 Second energy step, δE , calibration .................................................................................................................................14

7.6.1 Determining the EELS second energy step, δE ...................................................................................

2 14

7.6.2 Acquire E EEL spectrum ...............................................................................................................................15

CZLP

7.6.3 Obtain the value for CH between the zero-loss peak and peak E .............................

3 CZLP 15

7.6.4 Calculate calibrated energy step δE ..........................................................................................................

2C 15

7.7 Determining the calibrated EEL spectrometer energy resolution, ΔE .................................................15

7.7.1 Acquisition of a ZLP EEL spectrum ...............................................................................................................15

7.7.2 Obtain the value for CH for the zero-loss peak .................................................................................15

7.7.3 Calculate EEL spectrometer energy resolution, ΔE ........................................................................15

7.8 Record items ..........................................................................................................................................................................................16

8 Uncertainty for the measurement result of energy resolution .........................................................................17

Annex A (informative) Example of measurement procedure for energy resolution

determination ......................................................................................................................................................................................................18

Annex B (informative) Correspondence between energy values of XPS C1s and EELS carbon

K edge ............................................................................................................................................................................................................................26

Bibliography .............................................................................................................................................................................................................................28

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 23420:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 202, Microbeam analysis, Subcommittee

SC 3, Analytical electron microscopy.
A list of all parts in the ISO 23420 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 23420:2021(E)
Introduction

In order to understand the chemical composition, the atomic bonding and the electronic structure,

electron energy loss analysis is often performed with the scanning transmission electron microscope

or the transmission electron microscope (S/TEM) equipped with the electron energy loss (EEL)

spectrometer.

In the analysis using EEL spectrometer system, the energy loss of incident electrons by the inelastic

interaction via phonon and plasmon excitations, intra- and inter-band transitions and the inner shell

ionization can be measured. The inner shell ionization is particularly useful and important as it gives

the information on chemical composition of materials. For the precise analysis based on the energy loss

peak decomposition and its energy shifts, it is vitally important to understand the energy resolution

of the EEL spectrometer system. However, the determination method of the energy resolution is not

standardized yet.

This document provides the procedures for energy step calibration and energy resolution determination

useful for the electron energy loss spectrum analysis in the S/TEM equipped with the EEL spectrometer.

© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 23420:2021(E)
Microbeam analysis — Analytical electron microscopy —
Method for the determination of energy resolution for
electron energy loss spectrum analysis
1 Scope

This document specifies a determination procedure of energy resolution in the scanning transmission

electron microscope or the transmission electron microscope equipped with the electron energy loss

(EEL) spectrometer.

This document is applicable to both in-column type EEL spectrometer and post-column type EEL

spectrometer. These EEL signal detecting systems are applicable to a parallel detecting system and a

serial detecting system.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at https:// www .iso .org/ obp
3.1
beam diameter

full width at half maximum (FWHM) of the electron beam intensity profile for the STEM observation

3.2
Boersch effect

energy spread of electron beam due to Coulomb interaction (3.5) between electrons in the beam

3.3
channel
range of one pixel of the detector in the parallel detection (3.17) EELS
3.4
collection angle

EELS entrance aperture diameter divided by a camera length and a geometric factor (3.13) for the

STEM or the TEM diffraction mode, or EELS entrance aperture diameter divided by the distance from

crossover of the lens in front of the EEL spectrometer to the EELS entrance aperture for imaging mode

of the energy-filtering TEM
3.5
Coulomb interaction
repulsion of electrons by electric charge
3.6
detection plane
plane where energy dispersed electron focus
© ISO 2021 – All rights reserved 1
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ISO 23420:2021(E)
3.7
electron energy loss

energy shift of the electron kinetic energy due to the inelastic scattering in solids

3.8
energy dispersion

degree of change in position of the dispersed electrons at the detection plane (3.6) per unit energy change

3.9
energy resolution
FWHM of the zero-loss (3.21) peak
3.10
energy step

energy selecting window (3.11) per channel (3.3) in the parallel detection (3.17) EELS, or energy range

limited by the width of energy selecting slit in the serial detection (3.20) EELS

3.11
energy selecting window
energy range for selection of a specific energy loss value
3.12
entrance aperture
aperture for limiting the collection angle (3.4) of the EEL spectrometer
3.13
geometric factor

ratio of distance from a projector lens to an EELS entrance aperture to distance from the projector lens

to an image detection device
3.14
in-column type EELS
EELS system with the EEL spectrometer located in the imaging system of the TEM
3.15
irradiation diameter
diameter of the electron beam irradiation region for the TEM observation
3.16
K edge
energy loss related to K shell electron transition to the lowest empty state
3.17
parallel detection

simultaneous EELS signal detection for all energy-dispersed electrons focused on the detection plane (3.6)

3.18
plasmon-loss

energy loss of electron due to excitation of the quantized plasma oscillations of electrons

3.19
post-column type EELS

EELS system with the EEL spectrometer located behind the imaging/detecting system of the TEM

3.20
serial detection

EEL spectrum detection by scanning the dispersed electrons across the energy selecting slit in front of

the detector
2 © ISO 2021 – All rights reserved
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ISO 23420:2021(E)
3.21
zero-loss

unscattered and elastically scattered electrons (with only minimal loss of energy due to phonon

excitation), giving rise to an intensity peak or the position of which defines zero in the electron energy

loss spectrum
[SOURCE: ISO15932: 2013, 2.2.1.1]
4 Symbols and abbreviated terms
B spatial width of energy selecting window in the serial detection of the EELS
CCD charge coupled device
CFE cold field emission

CH sum of Ch (G, P) and the Ch (G, C-K). In the parallel detection system, CH is the number

1 1 1 1

of channels between the zero-loss peak and carbon K edge of graphite. In the serial detec-

tion system, CH is distance between the zero-loss peak and carbon K edge of graphite.

CH number of channels between the zero-loss peak [Figure 5, key 1] and the peak E

2 CZLP

[Figure 5, key 2] on the calibrated energy step δE in the parallel detection EELS. In

the serial detection EELS, CH is distance between the zero-loss peak [Figure 5, key 1]

and the peak E [Figure 5, key 2] on the calibrated energy step δE .
CZLP 1C

CH number of channels between the zero-loss peak and the peak E on the energy step

3 CZLP

δE in the parallel detection EELS. In the serial detection EELS, CH is distance between

2 3
the zero-loss peak and the peak E on the energy step δE .
CZLP 2

CH number of channels corresponding to FWHM of the zero-loss peak on the calibrated

energy step δE in the parallel detection EELS. In the serial detection EELS, CH is dis-

2C 4

tance between the zero-loss peak and the peak E on the calibrated energy step δE .

CZLP 2C

Ch (G, C-K) number of channels of the range from the graphite plasmon-loss (π + σ) peak [Figure 3,

key 1] to carbon K edge E [Figure 3, key 2] on the energy step δE in the parallel

C-K 1

detection system. In the serial detection system, Ch (G, C-K) is distance between the

graphite plasmon-loss (π + σ) peak [Figure 3, key 1] and carbon K edge E [Figure 3,

C-K
key 2] on the energy step δE .

Ch (G, P) number of channels of the range from the zero-loss peak [Figure 2, key 1] to the graphite

plasmon-loss (π + σ) peak E [Figure 2, key 2] on the energy step δE in the parallel

CZLP 1

detection system. In the serial detection system, Ch (G, P) is distance between the ze-

ro-loss peak [Figure 2, key 1] and the graphite plasmon-loss (π + σ) peak E [Figure 2,

CZLP
key 2] on the energy step δE .
CMOS complementary metal oxide semiconductor
CRM certified reference material
C1s carbon K shell binding energy of graphite measured by the XPS
D energy dispersion on the recording device of the EEL spectrometer
d sample thickness of the electron beam irradiated area
E value of electron energy loss such as plasmon-loss and ionization-loss

E measured plasmon-loss (π - π*) peak energy of boron-nitride under the condition of

BN-P
calibrated energy step δE
© ISO 2021 – All rights reserved 3
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ISO 23420:2021(E)
E position of noticed low-loss peak close to the zero-loss peak
CZLP
E carbon K edge energy in the EELS
C-K
EEL electron energy loss
EELS electron energy loss spectroscope/spectroscopy
FWHM full width at half maximum
GUM guide to the expression of uncertainty in measurement
m total number of available channels in the parallel detection of the EELS
n iteration number in acquisition of electron energy loss spectrum
RM reference material
STEM scanning transmission electron microscope/microscopy

S/TEM scanning transmission electron microscope/microscopy or transmission electron mi-

croscope/microscopy

s detector spatial resolution for the parallel detection. For the serial detection, s is slit

width of energy selecting window
TEM transmission electron microscope/microscopy
t acquisition time in acquisition of electron energy loss spectrum
XPS X-ray photoelectron spectroscope/spectroscopy
ZLP zero-loss peak
ΔE energy resolution
ΔE theoretical energy resolution
ΔE energy broadening

δE selected energy step in the first energy calibration. In the parallel detection system, δE

1 1

is selected from the preset value. In the serial detection system, δE is derived from the

energy width and its spatial width in the energy selection window
δE calibrated value of energy step δE
1C 1

δE selected energy step in the second energy calibration. In the parallel detection system,

δE is selected from the preset value. In the serial detection system, δE is derived from

2 2
the energy width and its spatial width in the energy selection window.

δE calibrated energy step of energy step δE by the second energy calibration step

2C 2

δE energy width of the energy-selecting window in the serial detection system of the EELS

λ mean free path of electron inelastic scattering
π π-bonding state
π* π-antibonding state
4 © ISO 2021 – All rights reserved
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ISO 23420:2021(E)
σ σ-bonding state
(π - π*) resonant oscillation of the π-bonding state and the π-antibonding state
(π + σ) resonant oscillation of the π-bonding state and the σ-bonding state
5 Definition of the energy resolution

The theoretical energy resolution ΔE is given from a convolution of an electron beam energy spread

[2]

and a spectrometer resolution. The theoretical energy resolution is shown as Formula (1) .

2 2 2 2
(ΔE ) ≈ (ΔE ) + (ΔE ) + (s / D) (1)
r 0 SO
where
ΔE is theoretical energy resolution
ΔE is energy spread of the primary electron beam
NOTE ΔE is affected by energy width of electron source and the Boersch effect.
ΔE is broadening of energy

NOTE ΔE is affected both the spectrometer focusing and the angular width of inelastic

scattering.

s is a detector spatial resolution for the parallel detection. For the serial detection, s is a slit

width of energy selecting window.
D is an energy dispersion of the spectrometer

In addition, acquisition time t and acquisition iteration number n influence the energy resolution ΔE .

Measurement of energy resolution ΔE is not easy because of the complicated formation of the EELS

system. It is well known that the full width at half maximum of the zero-loss peak is proportional

to the energy resolution ΔE . Actually, FWHM of the zero-loss peak is very often used as the energy

[3]

resolution . The energy resolution ΔE is also defined as FWHM of the zero-loss peak in this document.

6 Reference materials and energy determination
6.1 General

In order to determine the energy resolution of the EELS equipped in the S/TEM, it is indispensable to

calibrate the energy scale in advance. In this section, material selection for the energy scale calibration

and the procedure for determining the energy scale are described.
6.2 Materials selection for energy scale calibration

For the energy resolution determination, calibration of the energy scale is necessary. As an EEL

spectrometer cannot calibrate energy scale by itself, the reference material is necessary for the

calibration. Since the energy calibrated certified reference materials (CRMs) and/or reference materials

(RMs) are not available, it is necessary to select appropriate materials aiming to energy scale calibration,

as (internal) reference materials. The following characteristics are required for the material.

— Easy to obtain
— Easy to handle,
© ISO 2021 – All rights reserved 5
---------------------- Page: 10 ----------------------
ISO 23420:2021(E)
— Homogeneous,
— Stable,
— Having loss peaks at a low-loss energy region,

NOTE 1 For measuring the energy resolution, energy scale calibration is needed to perform within loss

energy region such as zero to several hundred electronvolt.
— Non-chargeable.

NOTE 2 In the first step of energy scale calibration on the EELS, loss energy known sample is needed. The

loss energy value is obtained by the XPS analysis of the sample. Non-chargeable material is needed for XPS

measurements.

In this document, graphite is recommended and used as a reference sample for the coarse energy

scale calibration. The other reference sample for the following fine energy scale calibration should be

selected from the materials, which has low-loss EELS peak, such as boron-nitride.

6.3 Binding energy measurement of graphite in the XPS

XPS C1s (carbon K shell binding energy) peak and EELS carbon K edge E are equivalent. The

C-K

correspondence of the energy values between XPS C1s and EELS carbon K-edge is described in Annex B.

XPS measurement of C1s peak shall be done about graphite standard sample with calibrated XPS

spectrometer.
[4]
The XPS shall be calibrated by ISO 15472:2010 .
7 Measurement procedure and energy resolution determination
7.1 General

In this subclause, the energy scale calibration of EELS and the procedure for determining energy

resolution are described. Annex A shows an example of actual measurement using this procedure.

A flowchart of measurement procedure is shown in Figure 1.
6 © ISO 2021 – All rights reserved
---------------------- Page: 11 ----------------------
ISO 23420:2021(E)
NOTE Numbers given in Figure 1 indicate corresponding clauses in this document.
Figure 1 — Flow chart of measurement procedure
© ISO 2021 – All rights reserved 7
---------------------- Page: 12 ----------------------
ISO 23420:2021(E)
7.2 Predetermination of binding energy
7.2.1 Obtain graphite and the other reference sample

Obtain a graphite and a second reference sample. The graphite reference sample has a Carbon K-edge

and an EEL peak E in an EEL spectrum. The second reference sample should have an EEL peak

CZLP

E appearing close to the zero-loss peak, but the peak spread should not overlap with the zero-loss

CZLP

peak. For example, the boron nitride plasmon-loss (π - π*) peak is suitable for the E as good as the

CZLP
graphite plasmon-loss (π + σ) peak.
7.2.2 Measure C1s of graphite by using the XPS

Measure the carbon K-shell binding energy of the graphite using XPS. The C1s is used as carbon K edge

energy E of graphite of the EELS. The XPS shall be calibrated by ISO 15472:2010.
C-K
7.3 Setup of the S/TEM and the EELS, and sample setting

Samples are recommended to be drop-cast onto a TEM grid, e.g. a holey carbon film support grid, to

ease the measurement. The electron microscope should be adjusted for S/TEM observation in advance.

EELS should also be adjusted in advance so that EEL spectra can be easily obtained. Parameters of the

S/TEM and the EELS are recorded as described in 7.8.

The quality of an EEL spectrum is easily affected by electron beam induced carbon contamination. The

use of an anti-contamination device is strongly recommended for suppression of the contamination.

Additionally, a clean environment in the column is also needed to minimise the contamination.

7.4 First energy step, δE , calibration
7.4.1 EELS acquisition set-up

Set the EEL spectrometer acquisition time and integration numbers so as to secure a sufficient

signal-to-noise ratio without any peak saturation for all measurements, including measurements in

subsequent sections. Since the measurement conditions such as irradiation current, entrance aperture

diameter and capturing device characterization like CCD / CMOS camera are different, acquisition time

and integration number cannot be uniformized. Therefore, users of this procedure should optimise

acquisition time and integration number for their own systems and should record these values.

7.4.2 Determining the EELS first energy step, δE

Set the energy step δE to cover all range from the zero-loss peak to the carbon K edge E of graphite.

1 C-K

In the parallel detection system, energy step δE is selected from the preset value under the condition

as Formula (2).
δE ≥ E / m (2)
1 C-K
where
E is carbon K edge energy in the EELS
C-K

m is total number of available channels in the parallel detection system of the EELS

8 © ISO 2021 – All rights reserved
---------------------- Page: 13 ----------------------
ISO 23420:2021(E)

In the serial detection system, set a full measurement range as the zero-loss peak to the carbon K edge

E of graphite. Energy step δE is obtained as Formula (3).
C-K 1
δE = δE / B (3)
1 S
where

δE is energy width of the energy-selecting window in the serial detection system of the EELS

B is spatial width of energy selecting window in the serial detection system of the EELS

7.4.3 Acquisition of carbon K-edge EEL spectrum

Irradiate a thin region of graphite with the electron beam and acquire an EEL spectrum including

the ZLP and the carbon K-shell edge. A d/λ (sample thickness / mean free path) ratio check should be

performed to ensure the thickness of the area irradiated is suitable for the procedure.

Since the intensities of the zero-loss peak, the plasmon-loss (π + σ) peak and carbon K edge E are

C-K

considerably different, spectra including all these peaks may need to be acquired separately.

If it is necessary to acquire separate EEL spectra, an EEL spectrum should be acquired to include the

zero- loss peak and the plasmon-loss (π + σ) peak, this region will give the value for Ch (G, P), (Figure 2).

Acquire a 2 spectra which includes the plasmon-loss (π + σ) peak and carbon K edge E , this region

C-K
will give the value for Ch (G, C-K), (Figure 3).

Furthermore, the sum of Ch (G, P) and the Ch (G, C-K) gives the value for CH (see 7.4.4).

1 1 1

Certain EELS systems are able to acquire Ch (G, P) and Ch (G, C-K) simultaneously if the EELS system

1 1
has a function capable of obtaining two regions of the EEL spectrum.
7.4.4 Calculate calibrated energy step δE

Obtain calibrated energy step δE as Formula (4). δE is the calibrated value of energy step δE .

1C 1C 1
δE = E / CH = E / (Ch (G, P) + Ch (G, C-K)) (4)
1C C-K 1 C-K 1 1
where

Ch (G, P) is the number of channels of the range from the zero-loss peak [Figure 2, key 1] to the

graphite plasmon-loss (π + σ) peak E [Figure 2, key 2] on the energy step δE in the

CZLP 1
parallel detection system. In
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 23420
ISO/TC 202/SC 3
Microbeam analysis — Analytical
Secretariat: JISC
electron microscopy — Method for the
Voting begins on:
2021­01­29 determination of energy resolution
for electron energy loss spectrum
Voting terminates on:
2021­03­26
analysis
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 23420:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2021
---------------------- Page: 1 ----------------------
ISO/FDIS 23420:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH­1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/FDIS 23420:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 3

5 Definition of the energy resolution ................................................................................................................................................... 5

6 Reference materials and energy determination ................................................................................................................. 5

6.1 General ........................................................................................................................................................................................................... 5

6.2 Materials selection for energy scale calibration ........................................................................................................ 5

6.3 Binding energy measurement of graphite in the XPS ............................................................................................ 6

7 Measurement procedure and energy resolution determination ....................................................................... 6

7.1 General ........................................................................................................................................................................................................... 6

7.2 Predetermination of binding energy .................................................................................................................................... 8

7.2.1 Obtain graphite and the other reference sample ................................................................................. 8

7.2.2 Measure C1s of graphite by using the XPS ................................................................................................. 8

7.3 Setup of the S/TEM and the EELS, and sample setting ........................................................................................ 8

7.4 First energy step, δE , calibration .......................................................................................................................................... 8

7.4.1 EELS acquisition set-up ............................................................................................................................................. 8

7.4.2 Determining the EELS first energy step, δE ............................................................................................

1 8

7.4.3 Acquisition of carbon K-edge EEL spectrum ........................................................................................... 9

7.4.4 Calculate calibrated energy step δE ............................................................................................................

1C 9
7.5 Measurement of peak close to the zero­loss peak, E , for the other reference
CZLP

sample using energy step δE ...................................................................................................................................................

1 12
7.5.1 EEL spectrum acquisition of the second reference sample using energy

step δE ...................................................................................................................................................................................

1 12

7.5.2 Obtain the value for CH between the zero­loss peak and the peak E ....................

2 CZLP 13

7.5.3 Calculate the peak E energy .................. .....................................................................................................13

CZLP

7.6 Second energy step, δE , calibration .................................................................................................................................14

7.6.1 Determining the EELS second energy step, δE ...................................................................................

2 14

7.6.2 Acquire E EEL spectrum ...............................................................................................................................15

CZLP

7.6.3 Obtain the value for CH between the zero­loss peak and peak E .............................

3 CZLP 15

7.6.4 Calculate calibrated energy step δE ..........................................................................................................

2C 15

7.7 Determining the calibrated EEL spectrometer energy resolution, ΔE .................................................15

7.7.1 Acquisition of a ZLP EEL spectrum ...............................................................................................................15

7.7.2 Obtain the value for CH for the zero­loss peak .................................................................................15

7.7.3 Calculate EEL spectrometer energy resolution, ΔE ........................................................................15

7.8 Record items ..........................................................................................................................................................................................16

8 Uncertainty for the measurement result of energy resolution .........................................................................17

Annex A (informative) Example of measurement procedure for energy resolution

determination ......................................................................................................................................................................................................18

Annex B (informative) Correspondence between energy values of XPS C1s and EELS carbon

K edge ............................................................................................................................................................................................................................26

Bibliography .............................................................................................................................................................................................................................28

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ISO/FDIS 23420:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non­governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 202, Microbeam analysis, Subcommittee

SC 3, Analytical electron microscopy.
A list of all parts in the ISO 23420 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved
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ISO/FDIS 23420:2021(E)
Introduction

In order to understand the chemical composition, the atomic bonding and the electronic structure,

electron energy loss analysis is often performed with the scanning transmission electron microscope

or the transmission electron microscope (S/TEM) equipped with the electron energy loss (EEL)

spectrometer.

In the analysis using EEL spectrometer system, the energy loss of incident electrons by the inelastic

interaction via phonon and plasmon excitations, intra- and inter-band transitions and the inner shell

ionization can be measured. The inner shell ionization is particularly useful and important as it gives

the information on chemical composition of materials. For the precise analysis based on the energy loss

peak decomposition and its energy shifts, it is vitally important to understand the energy resolution

of the EEL spectrometer system. However, the determination method of the energy resolution is not

standardized yet.

This document provides the procedures for energy step calibration and energy resolution determination

useful for the electron energy loss spectrum analysis in the S/TEM equipped with the EEL spectrometer.

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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 23420:2021(E)
Microbeam analysis — Analytical electron microscopy —
Method for the determination of energy resolution for
electron energy loss spectrum analysis
1 Scope

This document specifies a determination procedure of energy resolution in the scanning transmission

electron microscope or the transmission electron microscope equipped with the electron energy loss

(EEL) spectrometer.

This document is applicable to both in-column type EEL spectrometer and post-column type EEL

spectrometer. These EEL signal detecting systems are applicable to a parallel detecting system and a

serial detecting system.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at https:// www .iso .org/ obp
3.1
beam diameter

full width at half maximum (FWHM) of the electron beam intensity profile for the STEM observation

3.2
Boersch effect

energy spread of electron beam due to Coulomb interaction (3.5) between electrons in the beam

3.3
channel
range of one pixel of the detector in the parallel detection (3.17) EELS
3.4
collection angle

EELS entrance aperture diameter divided by a camera length and a geometric factor (3.13) for the

STEM or the TEM diffraction mode, or EELS entrance aperture diameter divided by the distance from

crossover of the lens in front of the EEL spectrometer to the EELS entrance aperture for imaging mode

of the energy-filtering TEM
3.5
Coulomb interaction
repulsion of electrons by electric charge
3.6
detection plane
plane where energy dispersed electron focus
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ISO/FDIS 23420:2021(E)
3.7
electron energy loss

energy shift of the electron kinetic energy due to the inelastic scattering in solids

3.8
energy dispersion

degree of change in position of the dispersed electrons at the detection plane (3.6) per unit energy change

3.9
energy resolution
FWHM of the zero-loss (3.21) peak
3.10
energy step

energy selecting window (3.11) per channel (3.3) in the parallel detection (3.17) EELS, or energy range

limited by the width of energy selecting slit in the serial detection (3.20) EELS

3.11
energy selecting window
energy range for selection of a specific energy loss value
3.12
entrance aperture
aperture for limiting the collection angle (3.4) of the EEL spectrometer
3.13
geometric factor

ratio of distance from a projector lens to an EELS entrance aperture to distance from the projector lens

to an image detection device
3.14
in-column type EELS
EELS system with the EEL spectrometer located in the imaging system of the TEM
3.15
irradiation diameter
diameter of the electron beam irradiation region for the TEM observation
3.16
K edge
energy loss related to K shell electron transition to the lowest empty state
3.17
parallel detection

simultaneous EELS signal detection for all energy-dispersed electrons focused on the detection plane (3.6)

3.18
plasmon-loss

energy loss of electron due to excitation of the quantized plasma oscillations of electrons

3.19
post-column type EELS

EELS system with the EEL spectrometer located behind the imaging/detecting system of the TEM

3.20
serial detection

EEL spectrum detection by scanning the dispersed electrons across the energy selecting slit in front of

the detector
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ISO/FDIS 23420:2021(E)
3.21
zero-loss

unscattered and elastically scattered electrons (with only minimal loss of energy due to phonon

excitation), giving rise to an intensity peak or the position of which defines zero in the electron energy

loss spectrum
[SOURCE: ISO15932: 2013, 2.2.1.1]
4 Symbols and abbreviated terms
B spatial width of energy selecting window in the serial detection of the EELS
CCD charge coupled device
CFE cold field emission

CH sum of Ch (G, P) and the Ch (G, C-K). In the parallel detection system, CH is the number

1 1 1 1

of channels between the zero-loss peak and carbon K edge of graphite. In the serial detec­

tion system, CH is distance between the zero-loss peak and carbon K edge of graphite.

CH number of channels between the zero­loss peak [Figure 5, key 1] and the peak E

2 CZLP

[Figure 5, key 2] on the calibrated energy step δE in the parallel detection EELS. In

the serial detection EELS, CH is distance between the zero­loss peak [Figure 5, key 1]

and the peak E [Figure 5, key 2] on the calibrated energy step δE .
CZLP 1C

CH number of channels between the zero­loss peak and the peak E on the energy step

3 CZLP

δE in the parallel detection EELS. In the serial detection EELS, CH is distance between

2 3
the zero­loss peak and the peak E on the energy step δE .
CZLP 2

CH number of channels corresponding to FWHM of the zero­loss peak on the calibrated

energy step δE in the parallel detection EELS. In the serial detection EELS, CH is dis­

2C 4

tance between the zero­loss peak and the peak E on the calibrated energy step δE .

CZLP 2C

Ch (G, C-K) number of channels of the range from the graphite plasmon-loss (π + σ) peak [Figure 3,

key 1] to carbon K edge E [Figure 3, key 2] on the energy step δE in the parallel

C-K 1

detection system. In the serial detection system, Ch (G, C-K) is distance between the

graphite plasmon-loss (π + σ) peak [Figure 3, key 1] and carbon K edge E [Figure 3,

C-K
key 2] on the energy step δE .

Ch (G, P) number of channels of the range from the zero­loss peak [Figure 2, key 1] to the graphite

plasmon-loss (π + σ) peak E [Figure 2, key 2] on the energy step δE in the parallel

CZLP 1

detection system. In the serial detection system, Ch (G, P) is distance between the ze­

ro­loss peak [Figure 2, key 1] and the graphite plasmon-loss (π + σ) peak E [Figure 2,

CZLP
key 2] on the energy step δE .
CMOS complementary metal oxide semiconductor
CRM certified reference material
C1s carbon K shell binding energy of graphite measured by the XPS
D energy dispersion on the recording device of the EEL spectrometer
d sample thickness of the electron beam irradiated area
E value of electron energy loss such as plasmon-loss and ionization-loss

E measured plasmon-loss (π - π*) peak energy of boron-nitride under the condition of

BN­P
calibrated energy step δE
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ISO/FDIS 23420:2021(E)
E position of noticed low­loss peak close to the zero­loss peak
CZLP
E carbon K edge energy in the EELS
C-K
EEL electron energy loss
EELS electron energy loss spectroscope/spectroscopy
FWHM full width at half maximum
GUM guide to the expression of uncertainty in measurement
m total number of available channels in the parallel detection of the EELS
n iteration number in acquisition of electron energy loss spectrum
RM reference material
STEM scanning transmission electron microscope/microscopy

S/TEM scanning transmission electron microscope/microscopy or transmission electron mi­

croscope/microscopy

s detector spatial resolution for the parallel detection. For the serial detection, s is slit

width of energy selecting window
TEM transmission electron microscope/microscopy
t acquisition time in acquisition of electron energy loss spectrum
XPS X-ray photoelectron spectroscope/spectroscopy
ZLP zero­loss peak
ΔE energy resolution
ΔE theoretical energy resolution
ΔE energy broadening

δE selected energy step in the first energy calibration. In the parallel detection system, δE

1 1

is selected from the preset value. In the serial detection system, δE is derived from the

energy width and its spatial width in the energy selection window
δE calibrated value of energy step δE
1C 1

δE selected energy step in the second energy calibration. In the parallel detection system,

δE is selected from the preset value. In the serial detection system, δE is derived from

2 2
the energy width and its spatial width in the energy selection window.

δE calibrated energy step of energy step δE by the second energy calibration step

2C 2

δE energy width of the energy-selecting window in the serial detection system of the EELS

λ mean free path of electron inelastic scattering
π π-bonding state
π* π-antibonding state
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ISO/FDIS 23420:2021(E)
σ σ-bonding state
(π - π*) resonant oscillation of the π-bonding state and the π-antibonding state
(π + σ) resonant oscillation of the π-bonding state and the σ-bonding state
5 Definition of the energy resolution

The theoretical energy resolution ΔE is given from a convolution of an electron beam energy spread

[2]

and a spectrometer resolution. The theoretical energy resolution is shown as Formula (1) .

2 2 2 2
(ΔE ) ≈ (ΔE ) + (ΔE ) + (s / D) (1)
r 0 SO
where
ΔE is theoretical energy resolution
ΔE is energy spread of the primary electron beam
NOTE ΔE is affected by energy width of electron source and the Boersch effect.
ΔE is broadening of energy

NOTE ΔE is affected both the spectrometer focusing and the angular width of inelastic

scattering.

s is a detector spatial resolution for the parallel detection. For the serial detection, s is a slit

width of energy selecting window.
D is an energy dispersion of the spectrometer

In addition, acquisition time t and acquisition iteration number n influence the energy resolution ΔE .

Measurement of energy resolution ΔE is not easy because of the complicated formation of the EELS

system. It is well known that the full width at half maximum of the zero-loss peak is proportional

to the energy resolution ΔE . Actually, FWHM of the zero-loss peak is very often used as the energy

[3]

resolution . The energy resolution ΔE is also defined as FWHM of the zero-loss peak in this document.

6 Reference materials and energy determination
6.1 General

In order to determine the energy resolution of the EELS equipped in the S/TEM, it is indispensable to

calibrate the energy scale in advance. In this section, material selection for the energy scale calibration

and the procedure for determining the energy scale are described.
6.2 Materials selection for energy scale calibration

For the energy resolution determination, calibration of the energy scale is necessary. As an EEL

spectrometer cannot calibrate energy scale by itself, the reference material is necessary for the

calibration. Since the energy calibrated certified reference materials (CRMs) and/or reference materials

(RMs) are not available, it is necessary to select appropriate materials aiming to energy scale calibration,

as (internal) reference materials. The following characteristics are required for the material.

— Easy to obtain
— Easy to handle,
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ISO/FDIS 23420:2021(E)
— Homogeneous,
— Stable,
— Having loss peaks at a low-loss energy region,

NOTE 1 For measuring the energy resolution, energy scale calibration is needed to perform within loss

energy region such as zero to several hundred electronvolt.
— Non­chargeable.

NOTE 2 In the first step of energy scale calibration on the EELS, loss energy known sample is needed. The

loss energy value is obtained by the XPS analysis of the sample. Non-chargeable material is needed for XPS

measurements.

In this document, graphite is recommended and used as a reference sample for the coarse energy

scale calibration. The other reference sample for the following fine energy scale calibration should be

selected from the materials, which has low­loss EELS peak, such as boron­nitride.

6.3 Binding energy measurement of graphite in the XPS

XPS C1s (carbon K shell binding energy) peak and EELS carbon K edge E are equivalent. The

C-K

correspondence of the energy values between XPS C1s and EELS carbon K-edge is described in Annex B.

XPS measurement of C1s peak shall be done about graphite standard sample with calibrated XPS

spectrometer.
[4]
The XPS shall be calibrated by ISO 15472:2010 .
7 Measurement procedure and energy resolution determination
7.1 General

In this subclause, the energy scale calibration of EELS and the procedure for determining energy

resolution­are described. Annex A shows an example of actual measurement using this procedure.

A flowchart of measurement procedure is shown in Figure 1.
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ISO/FDIS 23420:2021(E)
NOTE Numbers given in Figure 1 indicate corresponding clauses in this document.
Figure 1 — Flow chart of measurement procedure
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ISO/FDIS 23420:2021(E)
7.2 Predetermination of binding energy
7.2.1 Obtain graphite and the other reference sample

Obtain a graphite and a second reference sample. The graphite reference sample has a Carbon K-edge

and an EEL peak E in an EEL spectrum. The second reference sample should have an EEL peak

CZLP

E appearing close to the zero­loss peak, but the peak spread should not overlap with the zero­loss

CZLP

peak. For example, the boron nitride plasmon-loss (π - π*) peak is suitable for the E as good as the

CZLP
graphite plasmon-loss (π + σ) peak.
7.2.2 Measure C1s of graphite by using the XPS

Measure the carbon K-shell binding energy of the graphite using XPS. The C1s is used as carbon K edge

energy E of graphite of the EELS. The XPS shall be calibrated by ISO 15472:2010.
C-K
7.3 Setup of the S/TEM and the EELS, and sample setting

Samples are recommended to be drop-cast onto a TEM grid, e.g. a holey carbon film support grid, to

ease the measurement. The electron microscope should be adjusted for S/TEM observation in advance.

EELS should also be adjusted in advance so that EEL spectra can be easily obtained. Parameters of the

S/TEM and the EELS are recorded as described in 7.8.

The quality of an EEL spectrum is easily affected by electron beam induced carbon contamination. The

use of an anti-contamination device is strongly recommended for suppression of the contamination.

Additionally, a clean environment in the column is also needed to minimise the contamination.

7.4 First energy step, δE , calibration
7.4.1 EELS acquisition set-up

Set the EEL spectrometer acquisition time and integration numbers so as to secure a sufficient

signal-to-noise ratio without any peak saturation for all measurements, including measurements in

subsequent sections. Since the measurement conditions such as irradiation current, entrance aperture

diameter and capturing device characterization like CCD / CMOS camera are different, acquisition time

and integration number cannot be uniformized. Therefore, users of this procedure should optimise

acquisition time and integration number for their own systems and should record these values.

7.4.2 Determining the EELS first energy step, δE

Set the energy step δE to cover all range from the zero-loss peak to the carbon K edge E of graphite.

1 C-K

In the parallel detection system, energy step δE is selected from the preset value under the condition

as Formula (2).
δE ≥ E / m (2)
1 C-K
where
E is carbon K edge energy in the EELS
C-K

m is total number of available channels in the parallel detection system of the EELS

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ISO/FDIS 23420:2021(E)

In the serial detection system, set a full measurement range as the zero-loss peak to the carbon K edge

E of graphite. Energy step δE is obtained as Formula (3).
C-K 1
δE = δE / B (3)
1 S
where

δE is energy width of the energy-selecting window in the serial detection system of the EELS

B is spatial width of energy selecting window in the serial detection system of the EELS

7.4.3 Acquisition of carbon K-edge EEL spectrum

Irradiate a thin region of graphite with the electron beam and acquire an EEL spectrum including

the ZLP and the carbon K-shell edge. A d/λ (sample thickness / mean free path) ratio check should be

performed to ensure the thickness of the area irradiated is suitable for the procedure.

Since the intensities of the zero-loss peak, the plasmon-loss (π + σ) peak and carbon K edge E are

C-K

considerably different, spectra including all these peaks may need to be acquired separately.

If it is necessary to acquire separate EEL spectra, an EEL spectrum should be acquired to include the

zero- loss peak and the plasmon-loss (π + σ) peak, this region will give the value for Ch (G, P), (Figure 2).

Acquire a 2 spectra which includes the plasmon-loss (π + σ) peak and carbon K edge E , this region

C-K
will give the value for Ch (G, C-K), (Figure 3).

Furthermore, the sum of Ch (G, P) and the Ch (G, C-K) gives the value for CH (see 7.4.4).

1 1 1

Certain EELS systems are able to acquire Ch (G, P) and Ch (G, C-K) simultaneously if the EELS system

1 1
has a function capable of obtaining two r
...

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